1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying apparatus or an electrophotographic printer, and more particularly to a developing apparatus therefor and an image forming apparatus utilizing such developing apparatus.
2. Related Background Art
In the image forming apparatus of electrophotographic system such as a laser beam printer or a copying machine, the developing apparatus therefor employs powdered developer, namely toner. The toner is contained in a developing container, carried by developer carrying means onto a developer bearing member and borne on a developer bearing member. The developer layer is subjected to thickness regulation and is given a predetermined electric charge by a regulating member, and is then carried to a developing area where the developer bearing member and an image bearing member are mutually opposed, thus being used in the development of the electrostatic latent image formed on the image bearing member.
FIG. 14 shows a magnetic one-component developing apparatus as an example of the developing apparatuses. This developing apparatus is provided with a developing container 43 containing magnetic toner (not shown) constituting magnetic one-component developer, and the magnetic toner is negative insulating toner having an average particle size of 6.6 to 9.0 .mu.m. At the aperture of the developing container 43, a developing sleeve 40 constituting the developer bearing member and consisting of an aluminum pipe is rotatably provided with a gap of about 300 .mu.m to a photosensitive drum 1. The developing apparatus of this example is constructed to be compact, and the developing sleeve 40 is accordingly designed with a diameter of 12 mm. The surface of the developing sleeve 40 is finished with suitable roughness, in order to bear and carry the toner of a desired amount thereon.
Inside the developing sleeve 40, there is provided an inrotational magnet roller 42 of a diameter of 10 mm, having two sets of magnetic poles N, S in alternate manner. Above the developing sleeve 40, there is provided an elastic blade 41 for example, of urethane rubber, constituting a developer regulating member, which abuts against the developing sleeve 40 with an abutting (contact) pressure of about 8 gf/cm. Behind the developing container 43, there is provided a developer carrying member 15.
The contact pressure of the elastic blade 41 is represented by so-called extracting pressure. In the present specification, the contact pressure of the elastic blade is always represented by the extracting pressure, which is measured as shown in FIG. 15.
As shown in FIG. 15, a stainless steel thin plate 45a of a thickness of 25 .mu.m is folded and another stainless steel thin plate 45b of a same thickness is sandwiched therebetween. These plates are inserted between the developing sleeve 40 and the elastic blade 41 in contact therewith and the stainless steel thin plate 45b in the center is extracted by an unrepresented spring scale. The extracting pressure is determined by dividing the reading of the spring scale, when the stainless steel thin plate 45b in the center is extracted, with the width thereof namely the length across the extracting direction.
The magnetic toner contained in the developing container 43 is carried onto the developing sleeve 40 by the carrying member 15 and is supported on the surface of the developing sleeve 40 by the magnetic force of the magnetic roller 42. The toner thus supported is carried by the rotation of the developing sleeve 40 to the position of the elastic blade 41, where the thickness of the toner layer is regulated to an appropriate value by the elastic blade 41 maintained in contact with the developing sleeve 40 and an appropriate triboelectric charge (triboelectricity) is given by the friction between the developing sleeve 40 and the elastic blade 41.
The magnetic toner, thus adjusted in layer thickness and given the triboelectricity, is carried by the rotation of the developing sleeve 40 to the developing area opposed to the photosensitive drum 1, and is used for developing the electrostatic latent image formed thereon.
At the development, a developing bias voltage, consisting of superposed AC and DC voltages, is applied to the developing sleeve 40 by a high voltage source 44, and the toner on the developing sleeve 40 is deposited onto the latent image on the photosensitive drum 1, thereby developing the latent image, while repeating the reciprocating motion between the developing sleeve 40 and the photosensitive drum 1 as if continuing the jumping motions according to the potential change in the AC component of the developing bias. Thus the latent image is visualized as a toner image by developing.
The toner particles are recently made finer in order to achieve faithful image reproduction of the electrostatic latent image thereby improving the image quality, but it is found that the fine particle toner with the weight average particle size D4 of 6.5 .mu.m or smaller tends to result in a low image density when applied to the compact developing apparatus shown in FIG. 14, because such small sized toner is difficult to charge.
FIG. 16 shows the difference in the variation of the image density as a function of the number of copies, between the toner of a particle size of 6 .mu.m and that of 8 .mu.m.
The average particle size of the toner can be measured with various methods, but it is measured in the present specification with the Coulter Multisizer II (Coulter Electronics, Inc.), employing the following method.
Aqueous NaCl solution of about 1% is prepared as electrolyte, employing primary sodium chloride. (Also ISOTRON (R)-II is available as the commercial product from Coulter Scientific Japan Co.) 150 to 200 ml of the electrolyte is added with a surfactant, preferably 0.1 to 5 ml of alkylbenzene sulfonate salt, as the dispersant, and with 2 to 20 mg of the toner as the specimen to be measured. Then the electrolyte in which the specimen is dispersed is subjected to dispersion for 1 to 3 minutes by an ultrasonic disperser and to the measurement of the volume and number of the toner particles with the above-mentioned measuring apparatus with an aperture of 100 .mu.m, and the volume distribution and the number distribution are calculated. Then the weight average particle size D4 is calculated from the volume distribution (central value of each channel being taken as the representative value therefor).
Referring to FIG. 16, the toner of the average particle size of 8 .mu.m provides a generally high image density even to the latter phase of 2500 image formations, though the image density is somewhat lower in the initial phase of the image formations. On the other hand, the toner of the average particle size of 6 .mu.m provides a particularly low image density in the initial phase of image formations and a generally low image density even to the latter phase of the image formations.
As will be apparent from these results, the image density tends to become low and has to be improved in case the fine particle toner of an average particle size of 6.5 .mu.m or less is used in the compact developing apparatus.
In the developing apparatus shown in FIG. 14, an increase of the contact pressure (extracting pressure) of the elastic blade 41 to the developing sleeve 40 from 8 gf/cm to about 30 gf/cm improved the triboelectric charging ability of the elastic blade 41 on the fine particle toner, thereby giving a larger charge thereto and improving the low image density.
FIG. 17 shows the change in the initial density of the solid black image as a function of the contact pressure of the elastic blade. In order to obtain a satisfactory density in the solid black image even from the initial phase of image formation, it is necessary, as shown in FIG. 17, to maintain the contact pressure of the elastic blade 41 at 20 gf/cm or higher.
However, such high contact pressure of the elastic blade 41 causes the developing sleeve 40 of a small diameter and a small thickness to bend, whereby the developing sleeve 40 is positioned close, at the central portion in the longitudinal direction, to the magnet roller 42 positioned therein and comes eventually in contact therewith.
In the presence of contact with the magnet roller 42, the developing roller 40 slides frictionally on the magnet roller 42 in the course of rotation, thereby causing drawbacks such as noise generation and an increased rotation torque of the developing sleeve 40.
Thus, in order to prevent the low image density in the development with the fine particle toner of the average particle size of 6.5 .mu.m or less, the developing sleeve 40 has to be given a larger strength, for example, by increasing the diameter to about 16 mm and increasing the thickness, in order to withstand the high contact pressure of the elastic blade 41.
In such case, however, the developing apparatus inevitably becomes larger in dimension. Also it is difficult to satisfactorily achieve compactization, which is strongly requested in the process cartridge.